Restoration of skeletal defects or wounds such as femoral neck fracture, spine fusion and lost teeth is a common procedure. For example, over 500,000 hip prosthesis implantations, 250,000 spine fusion surgeries, and 500,000 dental implant surgeries are performed annually in the United States alone.
Titanium and its alloys, due to their high toughness and excellent biocompatibility, are widely used in medical implants such as joint prostheses, fracture fixation devices, and dental implants. Other materials commonly used in medical and dental implants, include cobalt chrome, polished zirconium, oxinium (zirconium oxide) and stainless steel. However, titanium and these other materials demonstrate poor ability to bond to bone chemically, and thus osteolysis and subsequent loosening of implants comprising these materials are common.
The performance of an orthopaedic implant can be influenced by the quality of the interface formed between the implant and bone or bone cement. The development of the implant-to-bone (or cement) interface relies on a number of factors including surface area, charge, topography, chemistry and contamination of the implant. The implant-to-bone interface is the surface of the implant which interfaces or lies adjacent the bone when implanted.
Various techniques are known to modify the implant-to-bone interface topography to enhance implant-to-bone integration. These techniques include plasma spraying and electrochemical anodising of the implant-to-bone interface surface. Problems associated with plasma spraying and electrochemical anodising include, the formation of an implant-to-bone interface which has low fatigue strength, demonstrates poor adherence to the implant, and suffers from degradation, delamination or cracking during long term implantation.
A commonly used technique for improving tissue ingrowth into orthopaedic implants is abrasive particle blasting of the implant surface, alternatively known as grit-blasting or sand blasting. This cost efficient process, imparts a micron scale surface structure by blasting abrasive particles on the implant surface. Such roughened surfaces have been shown to promote cell attachment and thus improved physical implant-to-bone bonding. Furthermore, the increased area of a roughened surface means that more cells can attach to the implant-to-bone interface which also improves implant-to-bone physical bonding. The implant having such a modified implant-to-bone interface demonstrates good osseointegrative properties even in poor quality bone.
However the technique of abrasive particle blasting can cause significant changes to surface topography by damaging the metal elements on the surface of the implant. The technique of abrasive particle blasting can also cause heterogeneity of the surface chemistry due to the presence of abrasive particles embedded in the surface of the implant. The presence of the abrasive particles contaminate the surface of the implant and adversely affect the quality of the implant-to-bone interface. Furthermore, the abrasive particles can detach from the surface of the implant, leading to increased wear on the bone, implant and implant site.
Additionally, a percentage of the embedded abrasive particles protrude from the surface of the implant causing localised micromotion, movement of the implant relative to the implant site, and disruption of tissue ingrowth in the surface of the implant. Up to 40% of the surface area of the grit blasted implant can become contaminated with abrasive particles which can lead to implant-to-bone interface problems, reduced bio-compatibility of the implant and inflammation of the area local to the implant.